EFFECT OF NOISE ON HEARING
Noise is commonly use to describe sound that are unwanted or unpleasant, in contrast to sound such as music or speech. Several text book, in fact, define noise as sound that is discordant and nonperiodic, probably because such sound often have such unpleasant qualities. The potential of noise to damage hearing, however, is entirely related to such physical properties as its intensity, the length of time over which subject are exposed to it, and its time pattern and is not related to wheather the sound is pleasant or not. It would therefore be more appropiate to use the general term sound in discussing the hazards to hearing. However, we will use noise to describe sound that may be damaging to the ear because this word has traditionally had negative connotations and thus will be identified more readly with health hazards.
Noise of intensity and duration sufficient to cause hearing impairment are usually assosiated with industry. However, since it is solely the physical characteristics of the sound that determine its potential for causing hearing loss, the origin of the sound by itself has no influence on the degree of risk its presents for hearing damage. Thus sound to which people are exposed during recreational activities may pose as great a hazard to hearing as noise assosiated with work activities (including military activities, where gunshot noise in particular poses a high degree of risk).
Individual variation insusceptibility to noise is great, and probably factors that are not yet known affect individual’s risk of noise-induced hearing loss. Thus, only the degree of risk (probability) for acquiring a hearing loss can be predicted on the basis of our present knowledge about how the physical characteristic of noise and time of exposure to noise affect hearing. The risk of noise induced-hearing loss is in proportion to the intensity and duration of the noise, with the risk of injuring one’s hearing increasing with the length of exposure. The character of the noise-continuous or transient (such a gunshot)- also plays a role. Thus different type of noise pose different degrees of risk of hearing loss, even though the overall intensity of the noises is the same impulsive sounds such as gunshots generally pose a great risk than continuous noise.1 The spectral composition of the noise and the pattern of exposure (constant intensity vs fluctuating intensity or noise interpersed with intervals of relative silance) are also important factors that affect the degree of risk of hearing loss.
The firs effect of expossing an ear to noise of a certain intensity for a certain period of time is a reduction in hearing (elevated auditory threshold). This reduction in hearing is greatest immediately after the exposure and decrease gradually. If the noise has not been too loud or the exposure too long, hearing will gradually return to its original level, a type of hearing loss known as a temporary threshold shift (TTS). If the noise is louder than a certain value or the exposure time is longer than a certain time, causing a permanent threshold shift (PTS). The time course of the change in hearing threshold is illustrated schematically in Figure 28-1.
While the TTS probably result from temporarily impaired function of the sensory cells in the inner ear, a PTS is associated with irreversible damage to these cell. This damage can be seen when the cells are examined histologically under high-power magnification.an example of such damage is illustrated in Figure 28-2, which shows a scanning electron micrograph of the sensory cells (hair cells) in the inner ear of a monkey before and after exposure to gunshot noise. Hearing in person with damage hair cells cannot be restored. Noise exposure may also damage neural structures in the auditory nervous sistem, but the exact nature and extent of this damage is poorly understood.
As has been noted, the intensity and duration of exposure to noise primarily determine the degree of permanent hearing damage caused by the noise, and the hazard to hearing increases as the intensity and length of exposure increase. The distribution of noise’s energy over the requency spectrum is also important: low-frequency sounds are considered to be less damaging than high-frequency sounds of the same physical intensity. Another parameter of noise important in determining its potential to cause hearing loss is whether it is continuous or impulsive in nature; impulsive noise is more likely to cause hearing loss than is continuous noise of the same intensity and spectrum.
Figure 28-1. schematic diagram ilustrating how noise can affect hearing. The graph shows the hearing loss (threshold sift) at 4000 Hz a certain time (horizontal axis) after noise exposure. Noise with an intensity below a certain value is expected to give rise to a temporary threshold sift (90 dB, 7 days curve), while a louder noise (100 dB, 7 days) result in a permanent threshold sift. A very intense noise (120 dB, 7 days) give rise to a considerable permanent sift in threshold. (Adapted from Miller J: J Acoust Soc Am 56:3, 1974, with permission.)
Because there is a great individual variation in susceptibility to noise-induced hearing loss, people who are exposed to exactly the same noise for exactly the same period of time may not suffer the same degree of hearing loss. Some people can tolerate high intensity noise for a lifetime and not suffer any substantial degree of hearing loss, but other people may acquire a substantial hearing loss from exposure to much less intense noise. Attemps have been made to estimate an individual’s susceptibility to PTS by the degree of TTS evidenced on exposure to a test sound that is not loud enough to cause permanent hearing loss. However, the result of these attempts have been rather discouraging, and it seems there is no correlation between susceptibility to PTS and the degree of TTS in any individual person. At the present time the only way to determine such individual susceptibility is to test at frequent intervals the hearing of workers exposed to loud noise.
Figure 28-2. Scanning electron micrographs of sensory cells (hair cells) from a small segment of the basilar membrane of a monkey. A. Normal hair cells. B. Hair cell after noise exposure (gunshot). (Courtesy Proffesor Hans Engstrom, Uppsala, Sweden.)
Figure 28-3. Threshold sift (hearing loss) at 4 kHz as a function of the total amount of noise exposure (emission value). Each point representa an individual person, and the solid lines are the mean values of the threshold sift. The emission value is Leq + 10 log (T), where Leq represents the sound level (measured wit A-weighting) that is exceeded during 2% of the exposure time T (in months). For example, exposure to 85-dB noise during 20 years of work corresponds to 85 + 10 log (20 × 12) = 85 + 10 log 240 = 85 + 24 = 109. the threshold sift given is the threshold sift measured minus the threshold sift assumed to be normal considering the age of the person (everyone experiances some hearing loss as part of the normal process of aging, which is called presbycusis). For continuous noise, Leq deviates only slightly from the A-weighted sound intensity, but for noise that contains transient or intemittent noise (i.e., noise that vary consideraply in intensity), the difference between these two values is great. The graph thus shows that the average hearing loss as a result of exposure to continuous noise with a sound intensity of 85 dbB (A) or 20 years is less than 5 dB at 4 kHz, but that a number of people experiance a 30-dB to 40-dB threshold sift. (From Burn W and Fobinson DW: Hearing and Noise in Industry. London, HMSO, 1970.)
However, the distribution of susceptibility to PTS in a large population is relatively well known. Figure 28-3 shows hearing losses at 4 kHz for a number of people as a function of the noise emission level.2 This measure combines the two characteristic of noise (duration and intensity) assumed to be of the greatest importance in defining its potential for harm but does not take into account the nature of the noise (time pattern or spectrum).
Although we do not know whoch factor determine susceptibility to noise, recent studies in animals have pointed to ward some factors that may predispose a subject to noise-induced hearing loss. For example, study in rats that werw genitically predisposed to high blood presure showed that these rats acquired a higher degree of hearing loss from noise exposure than did normal rats when boths groups werw exposed to noise for their entire lifetime.3,4 Although these findings have not been duplecated in humans, the result of some human studies support a relationship between high blood presure and hearing loss from noise exposure5 (see also p. 529). Alteration in cochlear blood flow may also affect susceptibility to noise.6 More recent animal studies have disclosed that activation of particular neural circuit in the brainstem (the olivocochlear bundle) may protect the ear from the noise-induced hearing loss.7 the results of these studies undeline the complexity of the way noise affects and gives rise to hearing loss; they also shed some light on several unexplained effects of the exposure pattern and nature of the noise in producing noise-induced hearing loss. Research along these lines is likely to provide knowledge that may lead within the foreseeable future to development of more efficient ways to assess an individul’s susceptibility to noise hearing loss.
NATURE OF NOISE-INDUCED HEARING LOSS
Hearing loss is measured in decibles* relative to normative average hearing threshold. Such baseline thresholds are obtained-
......................................................................................................................................................
*dB, the abbreviation for decibel, is a logarithmic measure, used here as a measure of sound pressure. One decibel is one tenth of a logarithmic unit (a ratio of 1 : 10). The reason for using a logarithmic measure of sound pressure to measure hearing threshold is that the subjective sensation of sound intensity is approximately related to the logarithmic of the sound pressure.
Figure 28-4. Average estimated hearing losses for industrial workers exposed to noise of a certain level for different lengths of time. [From Taylor et al: J Acoust Soc Am 38:113-120, 1965]
by measuring the hearing thresholds of young people who have had known exposure to noise. However slightly different standards for “normal” hearing are used in different parts of the world (American National Standard Institute [ANSI] in the United States8; International Organization for Standardization [ISO] in Europe9), which may need to be taken into account when the risk of noise-induced hearing loss is evaluated. The difference between the hearing threshold of an individual and the “standard” hearing threshold is known as the hearing level (HL) and is measured in decibels. When the HL is plotted in the vertical axis as a function of the frequency tested, the resulting graph is known as an audiogram. Usually, the hearing thresholds are determined only in the frequency range of 125 to 8000 Hz (8 kHz), despite the fact that a person with normal hearing can hear sounds in the frequency range of 18 to 20,000 Hz.
Examples of estimated hearing loss taken from data obtained in a study of workers in the weaving industry are shown in Figure 28-4 as “predicted” audiograms. The individual curves on this audiogram represent different durations of noise exposure in years. As seen, the estimated hearing loss is greatest in a restricted frequency range around 4 kHz, but with continuing exposure the frequency range of hearing loss widens and the magnitude of the loss increases. These results obtained in weavers are typical for those exposed to noise in various manufacturing industries where the noise tends to be a broad spectrum and continuous in nature. Although many hypotheses have been presented, we do not know why the greatest hearing loss resulting from exposure to the broadband noise that is common in industries occurs around 4 kHz. We do know, however, that the frequency distribution of hearing loss depends to some extent on the spectrum of the noise. When the hearing-damaging effect of a noise with a narrow spectrum (i.e., its energy is limited to a narrow range of frequencies) is studied, the hearing loss usually is greatest over a frequency range slightly above (about ½ octave) the range at which the notice has its highest energy. However, the precise relationship between the spectrum of a noise and the distribution of hearing loss it can cause is not fully known.
Since hearing loss induced by industrial noise usually first affects the hearing threshold at frequencies around 4000 Hz (and thus above the 300- to 3000-Hz range essential for perception of speech), a person who suffers from noise-induced hearing loss often does not notice the loss until it has reached a relatively severe level. However, hearing tests can easily reveal hearing loss may indicate that the person in question is particularly susceptible to noise-induced hearing loss, frequent testing of the hearing of workers exposed to noise is important. In fact, it is powerful way to identify people who are particularly susceptible to noise exposure before they acquire a hearing handicap.
Being unable to hear weak sounds is not the only way that the hearing of people with noise-induced hearing loss is impaired. The deterioration of sensory cells in the inner ear that is caused by noise exposure also leads to a change in the way which sounds are perceived. Thus, although a person with a noise-induced hearing loss may understand some sound through amplification (asking people to speak louder or using a hearing aid); the quality of the sound is impaired. Such a person may have difficulty understanding speech, even when the sound has been amplified properly. This aspect of noise-induced hearing loss makes hearing more difficult when the individual is in a noisy environment or in a place in which several people are speaking at the same time. Many people with noise-induced hearing loss are also severely troubled by ringing in the ears (tinnitus).
NOISE STANDARDS
To reduce the risk of noise-induced hearing loss, a number if recommendations of acceptable noise levels have been established and appear in the form of noise standards. Different countries have adopted slightly different standards, and the ways in which standards are enforced also differ. All presently accepted standards use a single-value measure of the noise level and the duration f the exposure to calculate the risk the noise represents for causing permanent noise-induced hearing loss. Some of these standards include correction factors regarding the nature of the sound, for instance, impulsive vs continuous sounds.
Establishing Noise Standards and Damage Risk Criteria
The maximal noise level and duration accepted in most industrial countries is either 85 or 90 dB(A),* for 8 hours a day, 5 days a week. In Europe the 85-dB(A) level is more common, whereas in the United States 90 dB(A) is the level stated by the Occupational Safety and Health Administration (OSHA), although certain measures have to be taken if workers are exposed to noise levels above 85 dB(A) (for a review of noise standards see Suter10)
Individual variation in susceptibility to noise-induced hearing loss makes it impossible to predict what hearing loss an individual will acquire when he or she is exposed to a certain noise. Therefore, at best standards merely predict the percentage of people in a population with normal hearing who will acquire less than a certain specified (acceptable hearing loss when exposed to noise no louder than a certain value. When evaluating the so-called noise standards it is important to keep this in mind11. Noise standards are thus based on the fact that a certain percentage of normal-hearing population will acquire a permanent hearing loss (threshold elevation) greater than a certain value averaged over certain frequencies. The “allowed” hearing loss and the frequencies at which it is measured vary among standards and have been modified at intervals.
_____________________________________________________________________*The (A) after dB indicates that the noise spectrum has been weighted to place less emphasis on low frequencies that on high frequencies because low-frequency sounds generally post loss risk for causing hearing loss than high-frequency sounds.
There has been a tendency to adjust these criteria downwards to allow for less permanent hearing loss. In the beginning of the era in which efforts were made to reduce (or prevent) notice-induced hearing loss, the acceptable hearing loss was defined as the level of hearing loss at which an individual begins to experience difficulty in understanding everyday speech in quiet environment. This definition was based on the American Academy of Ophthalmology and Otolaryngology (AAOO) guidelines for evaluation of hearing impairment, 13 which state the ability to understand normal everyday speech at a distance of about 1,5 m (5 ft) does not noticeably deteriorate as long as the hearing loss does not exceed an average value of 25 dB at frequencies 500, 1000, and 2000 Hz. On the basis of this, an average hearing loss of 25 dB at frequencies 500, 1000, and 2000 Hz was taken to be hearing loss that resulted in a just-noticeable handicap. This level of hearing loss was originally designated in United States at the level of handicap at which a worker was entitle to receive workman’s compensation for loss of earning power (it is puzzling that this degree of hearing loss was later designated as acceptable)
More recently, some controversy about what constitutes a hearing impairment has emerged. The AAOO had focused n average hearing loss at 500, 1000, and 2000 Hz,14 but the National Institute for Occupational Safety and Health (NIOSH) criteria document15 states that the ability to hear sounds at 25 dB below normal at 500, 1000, and 2000 Hz not sufficient to assure that speech will be understood might be adequate under the optimal conditions in which speech reception is usually tested for audiological purposes. It has therefore been advocated that the average hearing loss at 1000, 2000, and 3000 Hz be used as a basis for evaluating the effects of noise exposure, instead of at 500, 1000, and 2000 Hz. However, to date this definition has not won general acceptance.
The Environmental Protection Agency (EPA) recommended to OSHA that 25 dB average hearing loss at 1000, 2000, and 4000 Hz be accepted the standards for when difficulties in understanding normal speech begin.16 The justification for excluding the hearing level at 500 Hz from calculations of hearing handicaps due to noise exposure is that hearing loss at 500 Hz is more often the result of factors other than noise exposure, such as middle ear disease. The inclusion of the hearing level at 3000 Hz in a composite describing the ability to understand normal speech seems justified because hearing at 3000 Hz is important for speech perception.
It has also been suggested that the average hearing level of 25 dB be replaced by 22 dB, while keeping the same frequencies defined by the EPA (1000, 2000, and 4000 Hz).17 The reasoning behind this suggestion was endorsed by AAOO, which, however, maintained that a “low fence” of 25 dB was acceptable.18 In general, these changes in hw measured pure tone thresholds should be weighted to obtain a single number to best define a hearing handicap that affect speech communication have led to a more realistic way describing the handicap of hearing loss.
Only hearing loss induced by noise has been discussed as imposing a handicap. However, the total acceptable hearing loss is that induced by noise plus whatever is assumed to be result of other causes such as age (presbycusis). Presbycusis also varies greatly among individuals. Nevertheless, it is the total hearing loss that determines the degree of handicap. So although noise induced hearing loss may not be at a level regarded to be a handicap according to the noise standard, older persons may suffer a level of hearing loss that can be handicapping, despite not having been exposed to noise that exceeded the standard.
Present Noise Standards. In the United States, legislation that covers noise includes Federal Aviation Act of 1958, the 1969 Amandement of Walsh-Healy Public Contracts Act, the Occupational Safety and Health Act of 1970, the Noise Control Act 0f 1972, and the Mine Safety and Health Act 0of 1978. These acts require certain agencies to regulate noise.
In Europe, legislation in various countries on industrial noise limitation has largely been guided by recommendations made by the ISC. These recommendations have generally set an upper limit of acceptable noise exposure at 85 dB(A) for 8 hours a day, 5 days per week, but in the United States 90 dB(A) has been standard.
The ISO recommendation is based on the probabilities of acquiring a hearing loss of 25 dB, averaged for 500, 1000 and 2000 Hz, with exposure to noises of different intensities for variable lengths of time. According to ISO recommendations,9 as much as 10% of a population with initially normal hearing will acquire a hearing loss of 25 dB or more, averaged over frequencies 500, 1000, and 2000 Hz, after 40 years of exposure to noise at a level 0f 85 dB(A). For 90 dB(A) noise level, those experiencing hearing loss increases to 21%. These values are based on studies of workers in weaving industry, and research indicates that the number of people with noise-induced hearing losses may be higher in other industries. However, the risk of hearing impairment doubles when the daily average noise exposure of 85 dB(A) increases to 90 dB(A), regardless of which data are used (see Suter10). Many European countries have chosen 85 dB(A) as the upper limit of acceptable noise exposure for 8 hours.
Recently, proposals have been put forward to lower the standard in the United States from 90 dB(A) to 85 dB(A).19 Accordingly, a recent OSHA hearing conservation amendment states that a noise monitoring program is mandatory in environments where the daily average of noise level is 85 dB(A) or higher.20 The monitoring program must be so designed that it identifies people who are exposed to noise levels of 85 dB(A) (8-hours weighted average) or more. If people are exposed to noise levels s of 90 dB(A) or higher for 8 hours per day, measures must be taken to reduce the noise; and if the measures do not result in a reduction of noise level to at least 90 dB(A) or lower, workers must participate in a hearing conservation program and employers must make available to workers personal hearing protection devices (ear protectors) and conduct hearing tests at specified intervals during employment. If a hearing loss of 10 dB average over frequencies 2000, 3000, and 4000 Hz is detected, the person must be referred for further evaluation, and action must be taken to avoid further deterioration of hearing.
As early as 1972, NIOSH, in its criteria document,15 recommended that the 90 dB(A) permissible exposure limit be lowered to 85 dB(A). This recommendation also included suggestion about audiometrical testing, use of hearing protectors, notification of worker and how records should be kept. Only some of these suggestion were accepted by the U.S. Secretary of Labor, however, and the permissible exposure limit still remains at 90 dB(A), although audiometrical testing and certain other hearing conservation measures are now required when workers are exposed to noise above 85 dB(A). It has been advocated that noise standards be modified to reduce the number of people who acquiring hearing loss that can be regarded as a social handicap. The maximal tolerable noise level for 8-hour exposure is around 75 dB(A) if significant noise-induced hearing loss is to be eliminated.21 One of the main obstacles in adopting lower noise level such as the proposed 85 dB(A) was economic concerns: the cost of having all workplaces comply with such regulations was considered prohibitive. However, a much less expensive alternative, having all new equipment comply with regulations, was not even considered.
In the United States the EPA has regulated exposure to noise for the general population, deciding that no more than 5 dB hearing loss can be allowed at 4000 Hz as a result of environmental noise.22
The fact that present noise standards are based on a simplified measure ofnoise, dB(A), adds to the uncertainty in predicting the risk ofacquiring a hearing loss through exsposure to a certain noise. As discussed earlier, this single-valued dB(A) measure does not contain any information about the spectrum of the noise, nor does it include any information about whether the noise contains sharp transient sounds or sounds with other characteristics important in hearing loss (see pic. 523).
Relationship Between Noise Level and Exsposure Time. A conversion factor must be established to estimate the acceptablenoise level when the exsposure to noise is less than 8 hours per day.again the conversion factors differs throughout the world. Thus, in Europe a 3-dB doubling factor is commonly used to estimate how high a noise level is acceptable when the exsposure time to the noise less than 8 hours per day, but a 5-dB doubling factor is used in the United Astates. A 3-dBdoubling factor implies that a reduction of the exsposure time by a factor of 2 is equivalent to a reduction in the noise level by 3 dB. That is, when the exsposure time is reduced from 8 hours per day to 4 hours per day, this rule assumes that a sound level 3 dB higher would be acceptable. Or if the exsposure time to noise is 2 hours per day, a sound level 6 dB higher can beaccepted and so on. This rule reflects the equal energy principle, which assumes that the total energy of the noise determines the risk for permanent hearing loss. However, using a 5-dB doubling factor means that every time the exsposure time is reduced by a factor of 2, he noise level can be increased by 5 dBwithout increasing the risk of hearing loss. This implies that exsposure to a noise for less than 8 hours per day decreases the risk more than the equalenergy principle implies and, therefore, that a higher total energy can be tolerated when the exsposure time to noise is reduced.
The noise standard presently in effect in the United States (29 CFR 1910.95) states that the maximum time-weighted exsposure level acceptable is 90 dB(A) for 8 hours with a 5-dB trading relation (doubling factor) between exsposure time and intensity: this trading relation means that although the maximum acceptable noise level for 8 hours is 90 dB(A), the maximum for half the time (4 hours) is 5 dB more (95 dB(A)). This noise level for this time is considered to be as hazardous as a 2-hour exsposure to 100 dB(A), a 1-hour exsposure to 105 dB(A), a 30-minute exsposure to 110 dB(A), and a 15-minute exsposure to 115 dB(A).
Using a fixed doubling factorimplies that a worker should be equally safe from acquiring a hearing loss when exsposed to noises of the same total sound energy, independent of whether the noises are presented as short-duration high-level noises or long-duration low-level noises. The value of the doubling factor for which this is true is, however, in dispite. The results of recent research indicate that a doubling factor of5 dB may be adequate for relatively low noise levels but that a smaller doubling factor (3 dB, i.e., equal energy) more correctly reflects the hazards presented by noise of a high level.
The validity of the equal energy principle has been questioned because animal exsperiment have shown hat hearing loss progresses more rapidly than predicted in respon to loud, short-duration sounds. Thus, it seems that a doubling factor ofless than 3 dB should be applied when the noise level is above a certain value. In the United States, standards have been thightened accordingly by stating that no worker should be exsposed to continuous noise above 115 dB(A) or impulsive noise above 140 dB(A),thus setting a ceiling for acceptable combination of noise intensity and exsposure time.
Because the level ofnoise exsposure usually varies during a work day, noise exsposure is often described by its equivalent level (Leq), whih is defined as the level of a noise that has the same average energy as the noise measured during a work day. The equivalent level is measured by summing the total noise energy to which a person is exsposed and dividig it by the duration of exsposure. The calculation of this equivalent level assumes that the equal energy principle discussed above is valid.
MEASUREMENT OF NOISE
Measurements ofsound levels are usually made at a location where people work. However, several factors are thereby left in doubt. One isthe efect of the head and pinna on the sound that actually reaches the ear. These two structures amplify sounds within a rather narrow range of frequencies (between 2 and 5 kHz) by as much as 10 to 15 dB. If the noise contains much energy in that range, the level of sound that actually reaches the ear may be as muchas 10 to 15 dB higher than the actual reading on a sound level meter placed in the person’s location when the person is not present.
Particular measurement problems also arise when the noise is impulsive in natur. Since the ear requires 100 ms to integrate a sound to perceive its loudness, noise level meters in earlier times integrated sound over about100 ms to provide a reading that was in accordance with the perceived loudness of the sound. This integration time is appopriate when the level of the sound is measured to assess itsability to annoy the hearer. However, whennoise levels are measured for the purpose of assessing the risk they pose to hearing, a much shorter integration time should be used because the cochlea integrates sound energy over 2 to 3 ms. Since presently available sound level meters (so-called impulse sound level meters) have an integration time of 35 ms, they underestimate the peak intensities of impulsive sounds and thus the potential ofsuch sounds to affect the cochlea (see Bruel23).
Another important factor of sound measurement is the variation in noise level atdifferent locations. Usually, a person does not maintain one work position but walks around,making the average exsposure difficult to estimate. Noise dosimeters have beendeveloped to improved the accuracy of measurements of noise exsposure. These devices, worn by the person whose noise exsposure is to be measured, register the sound level near the earor sometimes at other locations on the body and integrate the energy over an entire working day. Noise dosimeters thus function similiarly to radiation monitors.
PREVENTION OF NOISE INDUCED-HEARING LOSS
Naturally, the preferred method to preventing noise-induced hearing loss is to reduce the noise in a workplace below levels associated with significant risk to the people who work there. This method has often been disputes because of its serious economic implications; however, when noise restrictions are implemented onnew equipment only, the economic consequences are small. Although rebuilding old machinery for the purpose of reducing noise canbe costly, in the construction of new machinery state-of the-art engineering canreduce noise without excessive economic consequences. As an example, over a decade ago a new, fast papermaking machine with a noise level of 85 dB(A) was constructed using known technology to replace old machinery with a noise level of more than 110 dB(A). In fact, the noise level did not exeed 82 dB(A) in many workplaces in which this new equipment was installed, and the total cost ofreducing noise was only 1.1% of the total cost of the plant.
When old equipment is to be converted, it may cost less than $1000 per worker to comply with 90 dB(A) standards and about $1600 per worker to comply with 85 dB(A) standards (see Bruce,25 Table 4, page 609; see also Suter10). Undeniably, the cost of noise reduction inseveral industries is greater than in the example given,but it is equally obvious that in many branches of industry the costs is less, and there may evenbe cases in which the cost would be negligible. For this reason,setting a noise standard at a certain dB(A) value may in some cases be counterproductive because it provides no initiative to reduce the noise level below the standard, even when substansial reductions in noise level, beyond that necessary to meet the standard, could be achieved at minimal cost and improve worker comfort and productivity immeasurably.
It has been claimed that the problem of noise-induced hearing loss can be solved by personal protection devices (ear defenders). However, wearing ear protectors for long periods maybe hot and inconvenlent, and they impair speech communication evenbeyond the impairment caused by the noisy environment. In addition, ear protectors make it more difficult for people to hear alarm signalas or other acoustical signs of danger.
Nevertheless, there are times when ear protectors are appropriate. Two types are in common use: earmuffs, which are attached to a helmet or headband, and earplugs. Earmuffs can be removed more easily than earplugs and are therefore bettersuited for intermittent use, for example, when people are walking in and out ofnoisy area (such as airports), whereas earplugs are most practical for people who spend long periods of time in noisy environment. The sound attenuation of different types of earplugs and earmuffs depends not only onthe type ofdevice but also on how well it fits their individual person. Even results of laboratory testing where more deal situations can be achieved show great variability. When the sound attenuation of various protective devices is measured in the laboratory, earmuffs attenuate more than earplugs do. However, when hearing loss is assessed in people using these two types of ear protectors, earmuffs are usually shown to be less efficient than earplugs, eventhough earmuffs attenuate sound more. This discrepancy maybe due to the way sound atenuation is measured, or possibly since earmuffs are easier to remove, they may not always be worn when indicated. In addition, earmuffs tend to lose some of their sound attenuating power over time, thus they become lessefficient in reducng noise-induced hearing loss. Earplugs may also offer more efficient protection to some individuals but les to others because earplugs may must fit into the ear canal,the size of which varies widely in people. The sound attenuating power of earmuffs is less dependent on the anatomy of the wearer.
These problems were studied recently in shipyards, where there is often a combination ofintense, relatively continuous noise and superimposed in palsive noise, thus presenting an extreme hazard to hearing. Whenworkers were divided into two groups according to the intensity of the noise to which they were exsposed, those in the low-intensity noise group suffered more hearing loss than did those in the high-intensity noise group. This surprising result is likely related to the workers’ differing habits of wearing ear protectors: many more workers exsposed to high-intensity noise than low-intensity noise wore ear protectors. Infact,because the numbers of workers in the high-intensity noise group who did no wear ear protectors was so low, no analysis could be made inthis group of the relative benefit of wearing ear protectors, but in the low-intensity noise group, 1.73 times more workers who did not wear protectors suffered hearing loss than did those who wore protectors.
If the noise level to which people are exsposed cannot be reduced below harmful level, than measuring hearing loss at frequent intervals becomes are important part of a hearing conservation program, and it is the only known method of identifying people who are especially susceptible to noise-induced hearing loss. Since a hearing loss usualy beging in the high-frequency range, above the frequences used for everyday speech, it may not be noticed at first by the sufferer. However, suha hearing loss can easily be detected using ordinary pure tone audiometry. Persons who show such deterioration of hearing at high frequencies may be made aware that they are beginning to acquire a noise-induced hearing loss. The progress of hearing deterioration can usually be halted by moving to a les noisy environment, thus preventingthe hearing loss from reaching levels atwhic it becomes a social handicap. Modern hearing conservation programs focus on identifying persons who risk acquiring such a hearing loss so that they can take stepsto avoid a handicap. Many more people could not doubt be spared such a decline in the quality of their lives if more would take these simple precautions.
EFFECTS OF NOISE ON OTHER BODILY FUNCTIONS
The effects ofnoise on bodily functions other than hearing are poorly understood. Noise exsposure has been reported to cause an increase in blood pressure and alterations in other important bodily functions such as changes (ususally increases) inthe secretion of pituitaryhormones. However, more experimental evidence needstobe gathered to determine what the exact effects are and to distinguish between acute and long-term effects of noise on these functions.
Alterations in the body’s immune reactions and an increase in sensitivity to epinephrine and norepinephrine of the vascular system have been reported. Although it is known that the acute effect of noise on autonomic reactions increases as noise intensity is increased, theeffect of the time pattern of noise onthis response is less well understood than is its effect on hearing we know little about the ability of noise to alter excretion of pituitary hormones over extendedperiods of time.
It has been known for a long time that workers in noisy industries have a higher incidence of peripheral circulatory problems and heart problems than do those who are not exsposed to such high level of noise. However, because many other adverse factors besides noise exist in industrial environments, it has been difficult to identify the results of noise exsposure alone.
The efect of noise on blood pressure perhaps has been most thoroughly studied but has not been fully elucidated. During acute exsposure to noise, blood pressure ususally increases. The effect of noise on this bodily function is assumed o be mediated by the autonomic nervous system. Some reports have shown that prolonged exsposure to noise had a lasting effect on blood pressure in monkeys. Inother studies no effect was found on the normal increases inblood pressure with age that occurs inrats with normal blood pressure atbirth nor in animals with hereditary high blood pressure (spontaneoously hypertensive rats) when such rats were exsposed to noise for heir entire lives. (Unexpectedly, however, the spontaneously hypertensive rats developed considerably greater degrees of hearing loss from exsposure to noise than did rats without this hereditary predisposition to high blood pressure.)
Some retrospective studies (e.g., Jonsson and Hansson) of the effects ofexsposure to noise on the blood pressure of industrial workers found that workers who were exsposed to industrial noise had higher systolic and diastolic blood pressure, but other studies (e.g., Sanden and Axelsson) found no relationship between noise-induced hearing loss and blood pressure inshipyard workers. However, shipyards workers who had the highest degress of noise-inducedhearing loss had the greatest increases in heart rate during work, although the increases were not correlated with noise level. Again, there is no evidence that such an increases in heart rate has any long-term effects. If the results of the above-mentioned experiments in spontaneously hypertensive rats can be applied to human, then the results of he study of hypertension reported by Jonsson and Hansson may have to bereevaluated: by using hearing loss as the criterion for degree of noise exsposure they may have inadvertently selected workers who were predisposed to hearing loss because of their hypertension and not vice versa, as was intended.
SOUNDS ABOVE AND BELOW AUDIBLE FREQUENCY RANGE (ULTRASOUND AND INFRASOUND)
Sounds not audible to humans because their frequencies are outside our audible range are known as ultrasounds and infrasounds. The range of hearing in humans is usually given as 16 to 20,000 Hz in young people. The high-frequency limit usually shifts downward with age at 50 years it averages 10,000 Hz, although there is great individual variation. While the high-frequency limit is well defined (the hearing threshold rises abruptly when that frequency is exceeded), threshold increases much more gradually at the lower end of frequency scale, and sounds with frequencies below 10 Hz may be audible if they are intense. There is no evidence to indicate that exposure to sounds that are not audible can damage the ear.
Ultrasounds are heavily attenuated when transmitted in air and therefore decreases rapidly in intensity with distance from the source. Although very high intensities of ultrasound can kill furred animals such as mice, rats and guinea pigs because of the buildup of heat by sound absorption in the fur, such an effect could not occur in humans because bare skin cannot absorb enough energy to cause damage.
Exposure to low-frequency sound (infrasound) of high intensity has lately been reported to cause variation diffuse symptoms such as headache, nausea, and fatigue. Although few controlled studies have been conducted, it is possible that exposure to such sound may have some effect on general bodily functions. The result of some recent experiments indicates that infrasounds may decrease blood pressure, possibly through stimulation of the vestibular part of the inner ear. However, there is no evidence that such sounds can be hazardous to hearing.
REFERENCES
- Price GR, Kim HN, Lim DJ, Dunn D: Hazard from weapons impulses: Histological and electrophysiological evidence. J Acoust Soc Am 85:1245-1254, 1989
- Burns W, Robinson DW: Hearing and Noise in Industry. London, Her Majestv`s Stationery Office, 1970, p 241.
- Borg E, Møller AR: Noise and blood pressure Effects of lifelong exposure in the rat. Acta Physiol Scand (Stockh) 103:340-342, 1978.
- Borg E: Noise, hearing, and hypertension. Scand Audiol (Stockh) 10:125-126, 1981a
- Jonsson A. Hansson L: Prolonged exposure to a stressful stimulus (noise) as a cause of raised blood pressure in man. Lancet 1:86-87, 1977.
- Axelsson A, Borg E, Hornstrand C: Noise effects on the cochlear vasculature in normotensive and spontaneously hypertensive rats. Acta Otolaryngol (Stockh) 96:215-225, 1983.
- Rajan R, Johnstone BM: Contralateral cochlear destruction mediates protection from monaural loud sound exposures through the crossed olivocochlear bundle. Hear Res 39:263-278, 1989
- American National Standard Institute (ANSI) Standard for Audiometrics, S3.6, 1969.
- International Organization for Standardization (ISO): Assessment of Occupational Noise Exposure for Hearing Conservation Purposes. (Recommendation R1999) 1971
- Suter AH: The development of federal noise standards and damage risk criteria. In Lipscomb DM (ed) Hearing Conservation in Industry, Schools, and the Military. London: Taylor & Francis Publishing Co., 1988, pp 45-66
- Kryter KD: Impairment to hearing rom exposure to noise. J Acoust Soc Am 53: 1211-1234, 1973.
- Møller AR: Noise as a health hazard. Ambio 4:6-13, 1975.
- American Academy of Ophthalmology and Otolaryngology (AAOO), Committee on Conservative of Hearing: Guide for Evaluation of Hearing Impairment, 1959
- American Academy of Ophthalmology and Otolaryngology (AAOO): Guide for Conservation of Hearing in Noise (rev. ed.). Rochester, Minnesota: Trans Am Acad Ophthalmol Otolaryngol (suppl), 1973
- National Institute for Occupational Safety and Health (NIOSH) Criteria for Recommended Standard: Occupational Exposure to Noise. Publication No. HSM 73-11001, 1972
- Environmental Protection Agency (EPA): Testimony of Alvin F, Meyer, Jr. at the public hearings on proposed standards for occupational exposure to noise (submitted to U.S Department of Labor, Occupational Safety and Health Administration as Exhibit 57 in docket OSH-011), 1973
- Suter AH: The Ability of mildly hearing impaired individuals to discriminate speech in noise. Washington, D.C.: U.S Environmental Protection Agency (EPA #550/9-78-100) and U.S. Air Force (#AMRL-TR-78-4) reports, 1978
- American Academy of Ophthalmology and Otolaryngology (AAOO), Committee on Hearing and Equilibrium, and the American Council of Otolaryngology, Committee on Medical Aspects of Noise: Guide for evaluation of hearing handicap. JAMA 241: 2055-2059, 1979
- Suter A: Essentials of noise regulations. Otolaryngol Clin North Am 21(3): 551-562, 1979
- Occupational Safety and Health Administration (OSHA): Occupational Noise Exposure: Hearing Conservation Amendment, Final Rule. Federal Register 48:9738-9785, 1983
- Kryter KD: Extra auditory effects on noise. In Henderson D, Hamernik RP, Dosaujh DS, Mills JHM (eds): Effect of Noise on Hearing. New York: Raven Press, 1976, pp 531-546.
- Environmental Protection Agency (EPA), Office of Noise Abatement and Control Information on Levels of Environmental Noise: Requisite to Protect Public Health and Welfare with Adequate Margin of Safety. Washington, D.C.: Environmental Protection Agency (EPA#550/9-74-004), 1974
- Bruel PV: Noise: Do We Measure It correctly? Naerum, Denmark: Bruel and Kjaer, 1975, p 40
- Møller AR: Noise as a health hazard. Scand J Work Environ Health 3:73-79, 1977a
- Bruce RD: The Economic impact of noise control. In Cantell RW (ed): Symposium on noise: Its effects and Control. The Otolaryngologic Clinics of North America Philadelphia: W.B. Saunders CO., 1979, pp 601-607
- Erlandsson B, Hakanson H, Ivansson A, Nilsson P: The difference in protection efficiency between earplugs and earmuffs. Scand Audiol (Stockh) 9:215-221, 1980
- Nilsson R, Lindgren F: The effect of long term use of hearing protectors in industrial noise. Scand Audiol (Stockh) (suppl) 12:204-211, 1980
- Edwards RG, Hauser WP, Moiscev NA, Broderson AB, Green WW: Effectiveness of earplugs as worn in the workplace. Sound Vib 12:12-20, 1978
- Nilsson R, Liden G, Sanden A: Noise exposure and hearing impairment in the shipbuilding industry. Scand Audiol (Stockh) 6: 59-68, 1977
- Welch BL, Welch AS: Physiological Effects f Noise. New York Plenum Press, 1970
- Osguthorpe JD, Mills JH: Non-auditory effects of low-frequency noise exposure in humans. Otolaryngol Head Neck Surg 90:367-370, 1982
- Adrinkin AA: Influence of sound stimulation on the development of hypertension. Clinical and experimental results. Cor Vasa (Prague) 3:285-293, 1961
- Peterson ES, Augenstein JS, Travis DC, et al: Noise raises blood pressure without impairing auditory sensitivity. Science 211:1450-1452, 1981
- Borg E: Physiological and pathogenic effect of sound. Acta Otolaryngol (Stockh) (suppl) 381:1-68, 1981 b
- Borg E: Noise-induced hearing loss in normotensive and spontaneous hypertensive rats. Hear Res 8:117-130, 1982
- Sanden A, Axelsson A: Comparison of cardiovascular responses in noise-resistant and noise-sensitive workers. Acta Otolayngol (Stockh) (suppl) 377:75-100, 1981
General Reference
Burns W, Robinson DW (eds): Hearing and Noise in Industry. London: Her Majesty’s Stationery Office, 1970
Hamernik RP, Henderson D, Salvi R (eds): New Perspectives on Noise-Induced Hearing Loss. New York: Raven Press, 1982
Kryter KD: The Effects of Noise ion Man, 2 edt. New York: Academic Press, 1985
Lipscomb DM (ed): Hearing Conservation In Industry, Schools, and the Military. Boston: Little, Brown & Company, 1988
Pickles JO: Physiology of the Ear, 2 edt. New York: Academic Press, 1988
Salvi RJ, Henderson D, Hamernik RP, Colletti V: Basic and Applied Aspects on Noise-Induced Hearing Loss. New York: Plenum Press, 1985
